A. Field of the Invention
The present invention relates generally to information communication networks and, more particularly, to communication networks by means of which a large number of remotely positioned controllable devices, such as circuit breakers, motor overload relays, lighting systems, and the like, may be controlled from a central or master controller over a common network line which may comprise either the existing AC power lines, or a dedicated twisted pair line, or in some instances a fiber optic cable.
B. Description of the Prior Art
Various communication and control systems have been heretofore proposed for controlling a group of remotely located devices from a central controller over a common network line. Control systems for controlling distributed electrical loads are shown, for example, in Miller et al U.S. Pat. Nos. 4,167,786, 4,367,414 and 4,396,844 and in the earlier patents cross referenced therein. In such systems a large number of relatively complex and expensive transceiver-decoder stations, each of which includes a microprocessor, are interconnected with a central controller over a common party line consisting of a dedicated twisted pair for bidirectional communication between the central controller and all transceivers. Each of the transceiver-decoder stations is also of relatively large physical size due to the fact that a substantial amount of hardware is required, in addition to the microprocessor, to receive and transmit signals. Also, both the hardware and microprocessor consume substantial amounts of power. In fact, in Miller et al U.S. Pat. No. 4,167,786 it is necessary to provide a powersaver mode in which the major portion of the circuitry at each remote station is denergized to reduce power consumption during intervals when load changes are not being actuated.
Each of the transceiver-decoder stations controls a number of loads which must be individually connected to a particular transceiver by hardwiring, these interconnections being quite lengthy in many instances. In such a system, all transceivers can initiate messages at any arbitrary time in response to control input from the associated switches. Accordingly, it is not uncommon for two or more transceivers to simultaneously sense a free common party line and begin simultaneous transmission. This requires a special bus arbitration scheme to cause all but one of the interfering transceivers to drop out of operation while permitting one selected transceiver to continue its data transmission. Also, in such a system transmission from the transceiver to the central controller is very limited and consists merely of an indication of a manually operable or condition responsive switch or analog sensors such as a thermistor or other analog sensing device. In the load distribution control system shown in the above referenced prior art patents, the arbitration technique is dependent on the impedance levels of the active and inactive states of the data line. If the data line becomes stuck in a low impedance state, due to the failure of one of the connected transceiver decoders, further communication over the network line is prevented until the malfunctioning transceiver is physically disconnected from the data line.
In the communication and control system described in the above identified Miller et al patents a message transmitted over the network includes a preamble portion of a minimum of four bits. These preamble bits comprise 50% square waves which are utilized by the transceiver decoders to permit a phase lock loop circuit in each transceiver to lock onto the received preamble bits. The use of a minimum of four bits to provide phase loop lockon reducing the overall throughput of such a system. Also, in order to capture the preamble bits it is necessary to provide the phase lock loop circuit initially with a relatively wide bandwidth of about 5 KHz and then narrow down the bandwidth after the phase lock loop circuit has locked onto the preamble bits. Such an arrangement requires additional circuitry to accomplish the necessary change in bandwidth. Also, the relatively wide bandwidth necessary to capture the preamble bits also lets in more noise so that the security and reliability of the system is reduced in noisy environments.